Hot-dip aluminum coated steel sheet products are roughly classified into two types. One is for use in applications where heat resistance is required, while the other is for use in applications where corrosion resistance is required. Generally, the former is called a Type I aluminum coated steel sheet, while the latter is called a Type II aluminum coated steel sheet. The Type I aluminum coated steel sheet contains in its Al coatings a small amount of Si, which serves, when the product is heated at elevated temperatures, to suppress development of a Fe-Al alloy layer, rendering the product heat resistant. Even with such Type I aluminum coated steel sheets, the service temperature of the products which have been commercially available is normally about 600.degree. C. or below. On the other hand the Type II aluminum coated steel sheet has practically pure Al coatings. When compared with Type I products, Type II products are more corrosion resistant but less heat resistant.
Such a hot-dip aluminum coated steel sheet or strip is usually produced by hot-dipping a cold rolled strip of an aluminum killed steel or rimmed steel as a steel substrate in a hot-dip aluminum coating bath. In a commercial scale production, a steel slab is subjected to the steps of hot rolling, descaling, cold rolling, annealing and hot-dip aluminum coating, and the last-mentioned steps of annealing and hot-dip aluminum coating are normally carried out by passing the cold rolled strip of the steel substrate through a so-called Senzimir type hot-dip aluminum coating line installed with an in-line annealing equipment.
Japanese Patent Publication No. 53-15454 corresponding to U.S. Pat. No. 3,881,880 proposes preparation of a strip of an aluminum killed carbon steel which contains about 0.03% to about 0.25% by weight of carbon and has an amount of titanium added sufficient to precipitate the carbon in the steel and to provide an excess of uncombined titanium ranging between about 0.1% and 0.3% by weight, and hot-dip coating of the so prepared base steel strip with aluminum. It is taught in this patent that all the carbon in the steel is precipitated as titanium carbide to leave substantially no carbon in solution in the steel, in other words a steel base which resembles pure iron is provided, and in consequence, when the aluminum coated product is heated at elevated temperatures, Al in the coating layers is liable to diffuse into the steel base, whereby the oxidation resistance of the surfaces of the steel base may be improved.
On the other hand automobile manufacturers have recently requested and are now requesting, as a material for automobile exhaust gas systems, hot-dip aluminum coated steel strips, which have, in addition to an improved oxidation resistance at elevated temperatures, an improved strength at elevated temperatures (for example a tensile strength of at least 13 kgf/mm.sup.2, preferably at least 15 kgf/mm.sup.2, at 600.degree. C.), and which may be substituted for expensive AlSl 409 and 410 stainless steels. The above-mentioned patent does not teach how to commercially advantageously produce a hot-dip aluminum coated steel strip which has the requested strength at elevated temperatures as well as the improved oxidation resistance taught in that patent.